1. Field of the Invention
The present invention relates to a lead forming apparatus and a lead forming method for a resin-sealed semiconductor device, and more particularly to a lead forming apparatus and a lead forming method for a flat package.
2. Description of the Background Art
A flat package is a kind of resin-sealed semiconductor device. Flat packages include an SOP (Small Outline Package) in which leads extend from two opposite sides of a package and a QFP (Quad Flat Package) in which the leads extend from all of four sides of the package. The flat package is a surface-mount package, and has leads whose end portions are bent in a substantial L-shape so as to be mounted flat on a surface of a predetermined position of a wiring board.
A lead forming method using a background-art lead forming apparatus will be discussed below, referring to FIGS. 17 to 21.
First, as shown in FIG. 17, a QFP 3 having unformed leads 1 is prepared. In FIG. 17, only the leads 1 extending from one pair of two opposite sides of a package body 2 are shown and the leads from the other pair of two opposite sides are omitted.
In a step of FIG. 18, the QFP 3 is mounted on a forming die 4. The forming die 4 has a concave portion to accommodate a lower surface side of the package body 2, and when the package body 2 is inserted therein, the QFP 3 is supported by base portions of the leads 1 (near the package body 2). Then, a holder 5 and forming punches 6 descend from above the forming die 4.
The holder 5 has a concave portion to accommodate an upper surface side of the package body 2, and when the package body 2 is inserted therein, the base portions of the leads 1 are held between the forming die 4 and the holder 5.
When the forming punches 6 fully descend, as shown in FIG. 19, the leads 1 are pushed to bend in accordance with the contour of the forming punches 6. Thus, forming of the leads is completed. Furthermore, the shape of the lead is called a gull-wing shape as resembles a gull wing.
As discussed above, in the lead forming method using the background-art lead forming apparatus, the leads 1 are pushed to bend by the forming punches 6 while the base portions of the leads 1 are held between the forming die 4 and the holder 5 (see FIGS. 18 and 19).
The problem of this method lies in a distortion of the package body 2 produced in a fabrication process of the QFP 3. FIG. 20 schematically illustrates the distortion of the package body 2 (after lead-bending process step). There are a variety of distortions of the package body 2, but for simple illustration, FIG. 20 exaggeratingly shows the package body 2 which is warped upward.
If the package body 2 is warped upward, an arrangement of leads 1 are accordingly warped as shown in FIG. 20. When the warped leads 1 are held between the forming die 4 and the holder 5 as shown in FIG. 18, the arrangement of the leads 1 are temporarily corrected to be a linear arrangement. Then, the temporarily-corrected leads 1 are pushed to bend by the forming punches 6 and the end portions of the leads 1 after bending are linearly arranged. However, when the base portions of the leads 1 are released from the forming die 4 and the holder 5, the leads 1 are warped again, and as a result, only some of the end portions of the leads 1 comes into contact with a plane.
FIG. 21 schematically illustrates the QFP 3 after lead-bending process step. In FIG. 21, the end portion of a lead L1 which is located in a substantial center portion of the package body 2 is in contact with the plane, but the end portions of leads L2 and L3 away from the lead L1, nearer to the corner of the package body 2, are out of contact with the plane. That is because the package body 2 is warped upward. The lead L3 furthermost from the plane is located endmost in a side of the package body 2. In general, the flatness of the leads is defined by a distance d between the plane and the end portion of the lead furthermost therefrom. Normally, the flatness is required to be not more than 100 .mu.m, or 50 .mu.m.
Since the lead out of contact with the plane may cause a junction failure in joining the lead with the wiring board, the problem of distortion of the package body 2 must be overcome, but has not been fully overcome until now. As the second best method, correcting the end portions of the leads in the lead-bending process step may be proposed, but the background-art lead forming apparatus and method have no construction or process step for the correction.
Further, in the lead forming method using the background-art lead forming apparatus, the leads 1 are bent by the forming punches 6 as shown in FIG. 18 and hence the forming punches 6 are brought into contact with surfaces of the leads 1. The surfaces of the leads 1 are generally solder-plated for soldering onto the wiring board. When the surfaces of the leads 1 come into contact with the forming punch 6, the solder-plate is removed and solder-plating residues like spikes are generated. The solder-plating residues may fall on the forming die 4 and the lead 1 with the solder-plating residues deposited thereon may be shipped. A more disadvantageous case is that the solder-plating residues which are deposited on the forming punches 6 ascend and descend again in a process for the next leads. The amount of solder-plating residues generated in one process is small, but becomes larger during a repeat of processes while the solder-plating residues are left on the forming punches 6. When the amount of solder-plating residues become larger to some extent, the solder-plating residues fall from the forming punches 6 to be deposited upon the leads 1 and may short-circuit between the leads 1, thereby causing a faulty operation.
Furthermore, in the lead forming method using the background-art lead forming apparatus, the angle of the end portion of the lead 1 to be formed is fixedly determined in accordance with the shapes of the forming punch 6 and the forming die 4. Therefore, changing the angle to be formed is not easy since it creates a need for changing the forming punch 6 and the forming die 4 themselves.